One type of a waveguide optical device having a buried diffraction grating includes a DFB laser made from compound semiconductors, for example.
in recent years, there are techniques to improve the laser characteristics of a DFB laser by varying the coupling coefficient, which determines the amount of feedback of a diffraction grating, along the direction of a cavity.
For example, there is technique to improve the stability of the longitudinal mode upon a higher power optical output, by reducing the coupling coefficient toward the center of the cavity, thereby reducing the spatial hole burning along the cavity.
There are other techniques to prevent occurrence of the spatial hole burning. For example, there is a technique to gradually reduce the width of the buried diffraction grating toward the center of the cavity. Furthermore, there is a technique to gradually reduce the height of the buried diffraction grating toward the center of the cavity.
in addition, there is a technique to increase the threshold gain difference or the gain difference between the main and side modes, using a structure wherein the coupling coefficient is increased at the center of the cavity but is reduced at ends, as compared to the center.
Furthermore, there are a techniques to narrow the spectral line width by increasing the length of the cavity, when a DEB laser is used as an FM modulation light source, by dividing the drive electrode into three parts along the direction of the cavity, and modulating the injection current of the center electrode.
There is a technique to improve sensitivity of a DFB laser against a light backward to a laser. In a DFB laser having upper and lower buried diffraction gratings with an active layer sandwiched therebetween, one of the buried diffraction grating is provided in the vicinity of the front end face of the laser so as to reflect reflected returning light of the laser light from the outside. In the structure having the buried diffraction gratings above and below the active layer, a precise fabrication while synchronizing the phases of the upper and lower diffraction gratings is difficult.